1. Field of the Invention
This invention relates to critical temperature indicators. More particularly, the invention relates to critical temperature indicators that provide an irreversible visual indication that the indicator has been exposed to a predetermined temperature, such as near the freezing point of water.
2. Description of the Prior Art
There are a variety of products that are temperature sensitive and when exposed to a predetermined temperature are negatively affected or rendered unusable. A few such products include foods, pharmaceuticals, blood, vaccines, paints, emulsions and beverages, for example. Thus, a critical temperature indicator can be beneficial when used with these products.
In some instances, it is important to know if the product has been frozen, while other times it is important to know if items are kept at a predetermined temperature in order that they may be preserved for use sometime later. The characteristics of the product may change to an extent that the product is useless if exposed to a critical temperature, e.g., freezing point for such product, or frozen and subsequently thawed.
Some freeze indicators have been provided which utilize the expansion characteristic of water to break a frangible ampule. These indicators can include a dye loaded pad which is triggered by water and indicates that the indicator has gone through a freeze stage.
Water based products freeze below the normal freezing point of water, i.e., 32.degree. F. (0.degree. C.), due to the super-cooling effect of water, which will permit water to stay in its liquid state substantially below its normal freezing point. For example, water may be cooled to as low as 3.2.degree. F. (−16.degree. C.) without freezing. Therefore, there is a need to sense freezing above that of normal freezing temperature.
This problem has been partially overcome by the addition of certain nucleating agents to the water. For example, cupric sulfide powder has been used as a nucleating agent to reduce the tendency of water to super-cool. Further improvements in this technology have introduced freezing point modifying additives, such as heavy water (D2O), thus the temperature at which the ampule breakage occurs is near the freezing point of water.
Another style of indicator utilizes volume reduction characteristics of organic compounds as they cool. A bulb and a portion of a capillary tube of a thermometer-like structure contain a colorless organic compound. Another organic compound saturated with a dye and which is miscible with the colorless compound, is located in the capillary tube and separated from the colorless compound by a solid, movable plug or a liquid which is immiscible with either the colorless or the dyed compound. The volume of the colorless compound upon cooling is less than the volume of the bulb so that the separating plug or liquid and at least a portion of the dyed compound are drawn into the bulb cooling. The dyed compound then mixes with the colorless compound to provide a visual indication that the predetermined temperature has been reached.
While these prior indicators have met some of the needs in the field, there remains a need to improve upon the technology. Further, there remains a need to reduce the cost of the indicator.